1. Field of the Invention
This invention relates to a magnetic disk unit, and particularly to a magnetic disk unit capable of making high precision reading and writing by magnetic heads with a thermal offtrack minimized.
2. Description of the Related Art
This type of magnetic disk unit generally has a magnetic head mounted on a rotary carriage and floats the magnetic head from the surface of a magnetic disk by an air pressure generated by the magnetic disk revolving at a high speed to read or write magnetic record from or to the magnetic disk.
A carriage of the magnetic disk unit is designed to read a positioning pattern written in the surface of a magnetic disk with a servo head and to form a servo loop for positioning.
Such a magnetic disk unit has been improved to enhance recording density. Methods for enhancing the recording density include a method to increase a track bit density and a method to increase a track density by reducing a track width. In these years, the track width has been reduced to about 10 micrometers.
FIG. 4 shows an example of a conventional magnetic disk unit. In FIG. 4, magnetic disks are rotatably supported on a base 43 with both ends of a disk rotating shaft 42 via a spindle hub 44 and bearings 45a, 45b. On the other hand, a plurality of magnetic heads 49 which are disposed to face the magnetic disks 41 are supported by an arm holder 52. The arm holder 52 is fixed to bearings 54a, 54b and rotatably supported by a head rotating shaft 53 via the bearings 54a, 54b. And, the magnetic heads 49 can be driven to rotate by a torque generated by a magnetic circuit 57.
The magnetic heads 49 consists of servo heads 48 used for positioning and data heads 47 for reading and writing data. The arm holder 52 has a plurality of arms 50 which support the data heads 47 and the servo heads 48. The servo head 48 faces a disk surface 41a set with a positioning track having a positioning data written. Data other than the positioning data is not written in the positioning track of the disk surface 41a. In the magnetic disk unit shown in FIG. 4, the bottom face of the third disk from the top is used as the disk surface 41a for setting the positioning track. It is optional wherever the positioning track is set and, the position of the servo head 48 is fixed accordingly.
In these years, with the spread of computers, their setting environments have become diversified. Therefore, a temperature in the environment where a computer is set may largely vary depending on when data is written or read. In such a case, there is a disadvantage that information cannot be read correctly because the position of the data head 47 moves out of the track which is positioned by the servo head 48, or the so-called offtrack takes place, due to thermal expansion of individual component members of the unit.
The causes of the offtrack are considered to be as follows.
Specifically, in a conventional magnetic disk unit, a material for the arm holder 52 supporting the servo head 48 and the data head 47 is aluminum or magnesium which is inexpensive and light, and has suitable strength. As to a material for the outer ring of the bearings 54a, 54b, ball-bearing steel or martensite-based stainless steel is used to enhance accuracy.
In case of contacting sections of the arm holder 52 and the bearings 54a, 54b, the inner diameter size of the arm holder 52 and the outer diameter size of the outer ring of the bearings 54a, 54b are increased due to thermal expansion when a temperature of the environment in which the magnetic disk unit is installed increases. But, the ball-bearing steel or martensite-based stainless steel which is the material for the outer ring of the bearings 54a, 54b has a coefficient of linear expansion which is about half of that of the aluminum or magnesium which is the material for the arm holder 52, so that the bearings 54a, 54b are not as largely deformed thermally as the arm holder 52. Therefore, the arm holder 52 is pulled in a direction (inward in the radial direction of the bearings 54a, 54b) that expansion is prevented by the bearings 54a, 54b, at a position that the bearings 54a, 54b are fixed. Thus, the arm holder 52 is deformed as a whole because distortion is applied in the same direction as the radial direction of the bearings 54a, 54b.
Accordingly, the arm 50, which is at a position where the arm holder 52 is in contact with the bearings 54a, 54b, is particularly largely deformed in the radial direction of the bearings 54a, 54b, i.e., in a direction causing an offtrack, as compared with the arm 50 at another position. Therefore, as shown in the thermal offtrack characteristics of FIG. 5, the data head 47, which is mounted on the arm 50 at a position to contact with the bearings 54a, 54b, is largely displaced, resulting in causing an offtrack.